This invention relates generally to the production of cathode ray tubes (CRT) and particularly to an apparatus for establishing the sealing length of the envelopes of such tubes.
A CRT for a color television set includes a glass funnel the wide end of which is closed off with a faceplate panel. The narrow end of the funnel curves through a contoured neck area into a cylindrical portion to which a cylindrical neck is attached. A phosphor screen, comprised of triads of different color emitting phosphors, is placed upon the inside surface of the faceplate panel. An electron gun is accurately positioned in the neck portion to generate three electron beams. The electron beams are attracted to the screen and impact the phosphors resulting the emission of a different color of light by each of the three phosphors. A visual picture is generated across the face of the screen by scanning the electron beams horizontally and vertically across the phosphor screen. The scanning is affected using a yoke which is accurately positioned on the contoured neck area of the funnel. The yoke is internally configured to conform to the contour of the contoured neck area of the envelope. The yoke magnetically deflects the electron beams horizontally and vertically to can the phosphor screen. Arranged between the electron gun and the screen is an apertured shadow mask through which the electron beams must pass. This shadow mask serves as a color selection electrode to cause each of the three electron beams to impact a phosphor of the proper color. The electron beams, therefore, are converged to cross at the shadow mask.
The convergence of the electron beams at the shadow mask, the coma, the yoke pull back and the electron beam focusing are determined by the distance of the yoke from the screen and also by the distance between the yoke and the electron gun. For this reason, a reference plane is established at a cross section of the contoured neck area where the yoke is positioned. This reference plane is normal to the longitudinal axis of the envelope and is used as a reference to measure the length of the neck. In the art this length is identified as the sealing length. The sealing length extends from the reference plane to the line on the cylindrical neck where the neck is cut off and sealed after the envelope is evacuated. The longitudinal position of the electron gun in the cylindrical neck also is referenced from the reference plane of the curved contoured neck area.
Establishing the sealing length of CRT's using a cross-sectional plane of the contoured neck area of the envelope as a reference position is known in the art. However, difficulties frequently arise because the dimensions of the envelope, and particularly those of the contoured neck area vary within allowable tolerances. For this reason, any equipment which relies on a single envelope dimensional to establish the reference plane is subject to undesirable sealing length variations because of the permissible dimensional tolerances of the envelopes.
The present invention overcomes this difficulty by the provision of an apparatus which utilizes the entire contoured neck area for establishing the reference plane from which the CRT sealing length is measured.